Time-of-flight mass spectrometer

ABSTRACT

A time-of-flight mass spectrometer with a pulsed ion source in which the flight path is symmetrical and contains at least two straight parts dimensioned so that velocity focusing as well as directional focusing takes place in the flight path.

United States Patent Poschenrieder Jan. 28, 1975 TIME-OF-FLIGHT MASS SPECTROMETER [56] References Cited l [75] Inventor: Walter P. Poschenrieder, UNITED STATES PATENTS Mintraching, Germany 2,768,305 10/1956 Wells 250/419 2,784,317 3/1957 R0bins0n.... 250/419 I 3,087,055 4/1963 Liehl 250/419 [73] Assigneez Max-Plan kl h f 3.576992 5/1971 Moormun 250/419 Foerderung der Wissenschailen e.V., Gmtingcm Germany FOREIGN PATENTS OR APPLICATIONS 193.775 5/1967 U.S.S.R 250/419 22 Filed: July 27 2 1,152,258 5/1969 Great Britain 250/419 Primary l;'.\aminerWi11iam F. Lindquist 1 1 pp N04 275,549 Atlurm'y, Agent, or Firm-Briscbois & Kruger 30 F A r t p t D t [57] ABSTRACT or a a J vorelgn pplca n S 0 A timc-of-flight mass spectrometer with a pulsed ion July Germany 2 source in which the flight path is symmetrical and contains at least two straight parts dimensioned so that ve- 1521 us. (:1 250/287, 250/294, 250/299 231 take [51] Int. Cl. HOlj 39/34 [58] Field of Search 250/419 TF, 41.9 ME 14 Claims, 4 Drawing Figures 1 TIME-OF-FLIGHT MASS SPECTROMETER A survey of known time-of-flight mass spectrometers is contained in the book by Erich W. Blauth Dynamic Mass Spectrometers published by Friedrich Vieweg & Sohn, Braunschweig 1965, pages 7l to 94.

Compared with other electronic mass spectrometers, time-of-flight mass spectrometers have the very often substantial advantage that almost all ions produced during a short-term ionising process can be analysed electronically and that a complete mass spectrum can be obtained at once. Compared to mass spectrographs with photographic registration they have the advantage of a much higher absolute sensitivity. Time-of-flight mass spectrometers are always superior to other mass spectrometers if a quick analysis, high absolute sensitivity, or a detailed check of single processes is required, as practically all other electronically registering mass spectrometers require a mass scan during which at a given moment only ions of a specified specific mass (mass/charge) are detected and all other ions are lost.

The resolution capability of time-of-flight mass spectrometers can be improved by special focusing measures. In the case of mass spectrometers this means ion optical measures by means of which the produced mass spectrum is rendered independent of the initial energy (energy focusing), or the initial momentum (momentum focusing), or more generally the initial velocity (velocity focusing") which are in the main produced by the thermal initial energy of the ions further of the direction of the ions leaving the ion source (direction focusing) or also ofthe location of ion production (space focusing). One refers to stigmatic focusing if one point of the ion source is again shown at one point in the ion detection arrangement, thereby not only resulting in radial but also in an axial focusing.

A time-of-flight mass spectrometer with space focusing and energy focusing under the name of Bendixtime-of-flight spectrometer (Blauth, Le. Page 65) and also a circular periodical time-of-flight spectrometer with speed and angle focusing under the name of Chronotron (Blauth, 1.c. Page 7 l) have become known.

Finally it is known that time dispersion resulting from the different kinetic initial energies of the ions can be eliminated in a time-of-flight spectrometer by combining a field-free straight flight path with a radial electric deflection field produced by a cylindrical condenser. This radial electric deflection field is so designed that the time dispersion in the straight flight path and the electric field are equal and opposite, thus producing velocity focusing. (Int. J. Mass Spectrom.lon Phys., 6 (1971) 291-295).

The use of known focusing time-of-flight mass spectrometers as mentioned in the next but last paragraph is limited by one or several of the following conditions:

a. The range over which the initial energies of the ions may extend must only be a few tenths of an electron volt,

b. The amount of the most probable initial energy of the ions must also amount to only a few tenths of an electron volt;

c. The ion source together with the acceleration system and the ion detection arrangement must be situated in or immediately adjacent to the deflection field, with which focusing is achieved;

d. The ion source must be activated by pulsed extraction fields.

These known focusing time-of-flight mass spectrometers can often not be used if the ions are produced by a corpuscular beam as for instance with a microbeam probe, or when pulsed extraction fields are a hindrance as is the case with coincidence time-of-flight mass spectrometers, or if due to some other practical reason the ion source together with the acceleration system and the ion detection arrangements have to be disposed at some distance from the focusing deflection field.

The time-of-flight mass spectrometer with velocity focusing, mentioned last has the disadvantage that only in the deflection field does radial direction focusing take place, while the straight flight-path is not included in the direction focusing process. Transmission (luminesity) between the entrance plane (ion source) and exit plane (ion detection arrangement) is consequently small and the sensitivity thus leaves much to be desired.

The present invention is thus based on the task to provide a time-of-flight mass spectrometer with velocity focusing as well as direction focusing which takes place over the whole of the flight path, with a corresponding high transmission and sensitivity and in which the ions to be analysed have high and widely spread initial velocities (that is initial energies or initial momentum).

Since velocity focusing is subject to much less stringent requirements with the present invention than with known time-of-flight mass spectrometers, many new areas of application emerge, for instance in connection with an ion microbeam probe, a field ion microscope atom probe or a coincidence operation time-of-flight mass spectrometer.

In developing this invention, a four-fold first order focusing is achieved, that is time velocity focusing (i.e. disappearance of time velocity dispersion in the image plane) an spatial velocity focusing (disappearance of lateral dispersion at the image plane), radial and finally axial focussing (stigmatic imaging of a point of the object plane in a point).

In the following some embodiments of the invention will be more particularly described with reference to the drawings, in which:

FIG. I shows a schematic representation of a time-offlight mass spectrometer in accordance with a first embodiment of the invention, the flight-path of which contains an electric deflection field;

FIGS. 2 and 3 are each a schematic representation of an embodiment of a time-of-flight mass spectrometer according to the invention, the flight-path of which contains magnetic deflection fields and FIG. 4 is a schematic representation of a further embodiment in which a slightly different solution was employed compared with the other embodiments.

The symbols and abbreviations as used in the following are listed in a table at the end of the description.

GENERAL.

In time-of-flight mass spectrometers one differentiates basically between two different types:

A. Time-of-flight mass spectrometers with acceleration of the ions by equal momentum incruments.

With this type the ions, which have been formed in an ion forming space or in a source space are accelerated in a pulsed electrical field, which must be as homogeneous as possible, since the ions may pass through different parts of the field during their acceleration.

B. Time-of-flight mass spectrometers with acceleration of the ions to equal energies.

Here the acceleration field does not have to be homogeneous as all ions will pass through the entire acceleration potential. Here there are various possibilities of forming a well defined ion parcel with the ion source. The simplest way is to use a pulsating ionising process, i.e., one produces the ions for example by means of a pulsed electron beam (see for instance Blauth, l.c. Section 5.222). The expansion of the ion parcel thus produced depends on the expansion of the ion forming volume calculated in the acceleration direction, on the duration of the ion forming pulses, and on the breadth of the spread of the initial energies.

In both types the resolution is limited decisively by the breadth of spread of the initial impulses or the initial energies, if no measures are taken for focusing, momentum or energy momentum of energies.

Besides the momentum and energy focusing, both of which can be combined under the heading ofvelocity focusing", direction focusing is to be achieved, as already stated. A further condition is that (due to the deflection caused by the focusing in the radial direction) no velocity dependent spatial dispersion should appear because of the deflection means effecting radial focus ing, as thus an extensive ion detection arrangement (e.g., ion capture means, influence collector, secondary electron multiplier) would be required. Finally only toroidal electrostatic sectional fields (see for example Z. Naturforschg. a, No. ll, 1955, 872-876) and non-homogeneous magnetic fields with circular beam axis, which can be produced by means of a magnet having a wedge-shaped air gap cut radially should be considered for the realisation of the present time-offlight mass spectrometers, since other fields do not appear to offer a significant advantage, when a stigmatic image is demanded. Furthermore, the additional condition should be satisfied in the first few embodiments which follow, that not only the ion source (or object plane) but also the ion detection arrangement (or image plane) is arranged at a certain distance from the deflection field boundaries and that apart from the deflection fields no additional lenses are required.

These conditions limit greatly the number of otherwise possible configurations, because it was found that a path, that is the arrangement of straight paths and deflection fields has to be symmetrical (axial or mirror symmetry) with regard to the centre line if no dispersion in time or in space is to take place, caused by the different initial speeds of the ions of predetermined mass, and if the entrance area of the ions into an entrance plane, which limits the flight path on the side of the ion source is to be at least radially represented in an exit plane which limits the flight path on the side of the ion detection arrangement.

If the ions are accelerated in the ion source to substantially equal energies, electrostatic deflection fields have to be used for focusing in order to avoid spatial mass dispersion.

Similarly, when accelerating ions to substantially equal momentum, it is necessary to work with magnetic deflection fields.

"Substantially equal" here means that each ion in the ion source is supplied with the same amount of energy 04 impulse, so that the kinetic energies or momentum of the ions entering the flight or drift path from the ion source only differ from each other due to their initial velocities present before acceleration.

Part of the relationships, which have to be fulfilled with the present time-of-flight mass spectrometer. are formula-wise the same in the cases of acceleration to equal momentum energies. only the field inhomogeneity parameters h and k are defined differently for the actual magnetic or electrical deflection fields.

For the magnetic field we have:

ila: lh)

Here n is the usual in-homogeneity factor, which contains the ratio between the pole shoe increase to the radius of the deflection path. ln particular, the width b of the air gap between the pole shoes in independent of the deflection angle due to the rotational symmetry, and it is in first approximation tied to n through the relationship Here 11,, means the width of the air gap at the point of the main beam path, and

u (r-r,,)/r,, the normalised radial distance from the main beam path.

For the electrostatic toroid field we have:

where c r/R is the so-called aspect ratio.

If the entrance plane (in which there is usually situated a circular or slit shaped mask, which divides the ion source from the flight path and limits the entrance area of the ions) should be imaged in the exit plane without velocity dependent local dispersion (achromatic"), (the exit plane usually having a slit shaped or circular mask, which divides the flight path from the ion detection arrangement) an if in the simplest case it is supposed that the symmetry plane or the radial intermediate image lies inside a single deflection field the following relationship has to be satisfied for the distance 3, between the entrance plane and the entrancesided field boundary as well as the distance g, between the exit-sided field boundary and the exit plane:

r g, ro/h g( lab/2) where here I is the sector angle of the field and thus the deflection angle.

In stigmatic imaging with an axial intermediate picture at the point of the radial intermediate image, then in addition to condition (3), the following condition has to be satisfied:

it will be seen that the conditions (3) and (4) are only fulfillable for k =11, i.e. for n V2 (magnetic field) or In this case n or c also depend on b.

The equations (3), (4) and (5) apply, as already mentioned, only for the simplest case, in which the symmetric plane or the radial intermediate image is situated inside the deflection field (see for example FIG. 1). It is, however, quite possible to split the deflection field into two symmetrical halves in such a way, that the plane of symmetry is situated in a field free space between the two field areas. It is also possible, to position several such systems one after the other (see FIG. 2), so that the exit plane of the preceding system coincides with the entrance plane of the following system. Also, symmetrical arrangements with alternating fields and- /or additional electrostatic or magnetic lenses are possible, in which case it is possible to work with parallel beam entry into the deflection fields. As in this process nothing basically new emerges, no further amplification is made.

Time-of-flight Mass Spectrometer with acceleration of ions to equal energies:

Example 1 This embodiment of the invention, which will be described with reference to FIG. 1, works with an ion source of known construction, in which the ions are accelerated to substantially equal energies. Since all the ions, apart from their initial energies lying within the energy spread AE, possess the same energy, an electric deflection field has to be used for focusing in order to avoid spatial dispersion of the ions because of their effective mass m, which would interfere with the desired spread due to the mass dependent time of flight. With an electrostatic deflection field conditions are somewhat different from those with a magnetic field, as an ion with higher energy, does not only travel on a wider path but also experiences slight deceleration in its path due to the higher potential present on this path. The deflection field is produced by a so-called toroid condenser 12. The parcels of ions separated in time and having an equal, but from parcel to parcel different mass are detected by an ion detection arrangement 14.

In a toroid field with a central path radius r,, and the deflection or sector angle 4 the time of flight r of an ion with the effective mass M (M ion mass/ion charge) and the energy E,,.(l B) is a g -h 74:

Time-of-flight dispersion I, in a straight flight path of length g equals:

In order completely to dispose of the total energy dependent time-of-flight dispersion, the energy dependent time-of-flight dispersion At in the deflection field must be equal and opposite to the energy dependent time-of-flight dispersion At in the straight flight path. This leads to the focusing condition of:

For a system working with radial imaging, which should not show any energy dependent time-of-flight dispersion between the entrance and exit planes. the following must apply with precondition (3):

In the centre of the deflection field a radial intermediate image 20 appears, through which passes also the plane of symmetry 21 of the system.

EXAMPLE 2:

This example corresponds to example 1 with the exception, that the entrance area of the ions into the flight path is imaged in the exit plane not only radially but also axially, that is stigmatically. In this example, apart from the appearance of the radial intermediate image, there will also be on the same spot an axial intermediate image.

With stigmatic focusing with an axial intermediate image and one obtains the condition:

tq/2 2 sin 1 3/2I (l2) By graphic solution of this equation one obtains the values:

gr gr 0 1 l99.2

EXAMPLE 3 This example corresponds to example 2 with the exception that no axial intermediate image appears at the point of the radial intermediate image and that the beam path, there observed in a radial direction, is parallel.

For stigmatic focusing without an axial intermediate image taking lue note of equation (5) the following equations result:

lg( n[(1/h I in(h (14) The solution can again be obtained graphically with the result:

c r /R 0.26. A time-of-flight mass spectrometer with these parameters is characterised by its handy geometry, since the straight flight paths and the deflection angle are comparatively small. The mass spectrometer has, as the mass spectrum is independent of the initial velocity or initial energy of the ions (prior to acceleration) and the entrance plane 16 is imaged in the exit plane 18 stigmatically with respect to flight time and image location, that is both radially as well as axially focusing action.

At the point of the radial intermediate image 20 (FIG. 1) the largest spatial energy dispersion occurs and it is possible there to limit the transmitted energy by means of a slit 22 (energy slit). With the last mentioned configuration the magnification V of the intermediate image is about 0.33 and the energy dispersion D at the point of the intermediate image is 0.76.

An ion, which enters or leaves the electric deflection field outside the centre path experiences at the field boundaries 24 or 26 an acceleration or deceleration. A calculation of these effects showed that with the required symmetrical beam path no disturbance of timeof-flight compensation results in the first order. The stray field at the boundaries of the toroid condensator solely results in a minimal correction of length g of the effective straight flight path.

Time-of-Flight Mass Spectrometer with acceleration of ions to equal momentum.

Here equations (Ia), (lb) and (1c) apply in combination with equation (3). If stigmatic imaging is desired, equations (4) or (5) also have to be satisfied.

As focusing condition, equation (9) which applies to the case of acceleration to equal energies is replaced by the following focusing condition:

EXAMPLE 4 For a time-of-flight mass spectrometer with a single sector shaped magnetic field, stigmatic imaging and axial intermediate imaging at the point of the radial intermediate image at one half of the deflection angle, the following equation must be satisfied:

(V 2/4) 1 sin( V2/2) I =-tg( V2/4) (16) This equation can be derived from equations (3) and (4) and (15) if one also considers that n V2 and h k v7.

The solution of equation (16), which for instance can be arrived at graphically, is:

gr= r The time-of-flight mass spectrometer in accordance with Example 4 can be realised for example with the following numerical values:

Central path radius r,, ID cm Length x of flight path I29 cm Acceleration field strength F I V/cm Duration of impulse t. 10 sec Length of acceleration path for the ions 4.2 cm Magnetic Induction B, at the point of the central path radius r,, GauB EXAMPLE 5 This example corresponds in the main to Example 4 with the exception that at the point of the radial intermediate image no axial intermediate image appears but the beam trajectories run parallel there as seen perpendicular to the axis of the sector shaped magnetic deflection field. Here, apart from equation l5), equation (5) must be considered instead of equation 4) and one obtains the following equation:

The solution of this equation, for example obtained by graphical means, is:

Examples 4 and 5 can be realised easily in practice and have each their own advantages. With the time-offlight mass spectrometer in accordance with Example 4, a stigmatic intermediate image appears, which enables an exact definition of the transmitted velocity range of the ions. Resolution is high, the deflection radius small, the flight path is however comparatively long.

The time-of-flight mass spectrometer of Example 5 is characterised by a shorter flight path and thus shorter distances between ion source or ion detection arrangement on the one hand and deflection field boundaries on the other. Thus for a given size of the magnets producing the deflection field a more compact assembly and larger transmission (greater luminosity) can be attained than in Example 4.

Examples 4 and 5 are of course not the only realisation possibilities of the time-of-flight mass spectrometer according to the invention with magnetic deflection fields, but depict solely the simplest embodiment. It is obviously quite possible to devise other embodiments which contain more magnetic sector fields between which are situated straight sections of the flight path. In all cases, however, the configuration has to be symmetrical with regard to a plane or an axis (corresponding to a double axis in crystallography) and at least one intermediate image in one radial area must be in existence.

The straight-line parts of the flight path can also contain ion-optical lenses in order to make a greater flexibility possible in design.

In what follows will be described two further especially interesting embodiments:

EXAMPLE 6 The time-of-flight mass spectrometer as illustrated in FIG. 2 consists of two identical parts. which may correspond to a mass spectrometer according to Example 4 or 5. Both parts are, however, arranged in such a way that the deflection of the ions in both parts is effected in opposed angular directions.

The first part is situated between an exit slit 31 of an ion source (not shown) this exit slit 31 forms the entrance slit of the time-of-flight mass spectrometer and a slit 33, which forms the exit slit of the first part and at the same time the entrance slit of the second part. The second part is situated between the slit 33 and a slit 34, which is formed by the entrance slit of an ion detection arrangement (not shown). The first part contains a magnetic sector field 37, the second part a magnetic sector field 39. The sector field 37 contains. at the point of the radial intermediate image, a shutter 46. which limits the range of momenta of the captured ions (and thus the extent of the initial velocity of the ions).

EXAMPLE 7 The schematically illustrated time-of-flight mass spectrometer shown in FIG. 3 is a development of the time-of-flight mass spectrometer according to Example 4. It contains two magnetic sector fields 38. 40. which each have a sector angle of 270 for example. The distance g, or g, between an exit slit 30 (of an ion source not shown) or an entrance slit 32 (of an ion detection arrangement not shown) on the one hand and the adjacent boundaries 34 or 36 of the first or second deflection fields 38 or 40, is:

gr= gr o/ m (h l (is) The length g of the straight line flight path between the exit side boundary 42 of the deflection field 38 and the entrance side field boundary 44 of the deflection field 40 is g (2r cos (h I )l/h sin(h l 19 The radial focusing condition is In the deflection field 38 at the point of the radial intermediate image a shutter 46 (momentum slit) is situated, which limits the range of momenta of the ions allowed to pass and thereby the initial velocity range of the ions.

An axial intermediate image appears at the point 48, that is at the half-way mark of the straight-line flight path between the two deflection fields.

The Examples 6 and 7 have the added advantage, that the finite width of the entrance slit 3] or 32 has no influence in the first approximation on the resolution.

The embodiments described for electrostatic deflection fields can of course be realised in a corresponding manner with magnetic fields or vice versa, as can be seen from the explanations given under the heading of General.

On transfer of Example 7 to the case of the acceleration of ions to equal energies, the following equation is applicable instead of equation (20);

The task of creating a time-of-flight mass spectrometer with velocity focusing, in which directional focusing over the whole system is provided, can be solved by at least one other way than the one described above. To explain this other solution reference is made once more to the last mentioned known time-of-flight mass spectrometer.

The known time-of-flight mass spectrometer has a single straight flight path and a single, only radial focusing electrostatic deflection field, which is produced by a cylinder condenser. The focusing conditions for the part of the beam path in the deflection field can be written as follows:

This equation corresponds to the above mentioned equation (9).

Furthermore, there applies:

g,= g, (r /h) tg (180 [MP/2]) (23) The above mentioned embodiment of the invention with electrostatic deflection fields, in which only radial focusing takes place, differ from the known time-offlight mass spectrometers in that g, (and 3,) are not equal to zero.

A further solution of the problem as stated at the beginning, that is to attain directional focusing over the whole system inclusive of the straight part of the flight path, consists in case B of the acceleration of ions of 10 equal energies only inasmuch as equations (22) and (23) as in the known case are complied with and that additionally also equation is satisfied. Also the straight part of the flight path must contain at least one ion optical lens. for example an Einzel lens, which images the entrance slit of the mass spectometer stigmatically in the entrance plane, of the electrostatic deflection field, which in this case consists of a nonhomogeneous toroid field.

An embodiment. in which equations (22), (23) and (24) apply is illustrated schematically in FIG. 4.

To the entrance slit 30 is connected a single straight part of the flight path, which length is indicated by g. In the centre of the straight part is an Einzel lens 50, which images the entrance slit 30 stigmatically on an intermediate slit 52, which latter divides the straight part of the running path from the deflection field. This deflection field is produced by a toroid condenser 12'. The beam path in the toroid condenser 12 is symmetrical, and in the plane of symmetry an energy slit 22 is situated as in the example of FIG. 1.

The exit slit 32 of the mass spectrometer is situated immediately at the boundary of the electric deflection or sector field. In front ofthe entrance shutter 30 or behind the exit slit 32 is an ion source 10 or an ion detection arrangement (not illustrated).

It is easily possible to partition the straight part of the flight path into two parts, of which one is situated between the entrance slit 30 and the toroid condenser 12 and the other between the latter and one exit slit, arranged at a distance from the condenser of the exit slit. Of course, in such a case both straight parts should each receive an imaging lens.

In the practical embodiment according to H6. 4 the following applied:

The last described solution can also in principle be used in the case of the acceleration of ions to equal momentum. The realisation possibilities are however problematic as the sector angle will be larger than 360 for the required magnetic field in this case. A beam path staying in one plane is thus not possible.

In conclusion it should be noted that the flight path in which compensation of velocity dispersion (that is velocity focusing) is effected, does not have to be exactly equal to the distance between object point and image point, that is the portion in which an image (direction focusing) appears. One can, for example, for direction focusing, use a certain piece in front of the entrance slit or behindthe exit slit. The beam crosssection is thus somewhat larger at for instance the point of the ion detection arrangement than in the image plane, but this does not matter in practice. Similar comments also apply to the ion source.

I claim:

1. Time-of-flight mass spectrometer comprising means defining an ion flight path having an entrance plane and an exit plane, a pulsed ion source beyond the entrance plane, which source emits a parcel of accelerated ions into said flight path, in which the ion parcel divides into partial parcels of ions of equal effective mass, with the ions of different partial parcels having differing effective mass, said flight path including a straight part, field generating means defining a deflection field which is traversed by the ions in substantially arcuate paths and which is dimensioned with regard to the straight part in such a way that the dispersion in the time of flight of the ions in the deflection field, caused by different initial velocities of ions of equal effective mass, is equal but reverse in sign to that in the straight path, and ion detection means located beyond the exit plane of the flight path, wherein the improvement consists in that the flight path is symmetrical with respect to a plane of symmetry mid-way between the entrance plane and the exit plane, said path including at least two said straight parts adjoining opposite boundaries of the deflection field, and an apertured stop located at an intermediate image plane in said plane of symmetry in the deflection field, said field generating means defining two deflection field areas symmetrically on opposite sides of said plane of symmetry, which areas are traversed by ions on opposite respective sides of a central arcuate path passing through said stop aperture, and means converging the ions to an image in the exit plane under conditions of both velocity and direction focusing.

2. Time-of-flight mass spectrometer according to claim 1 wherein the intermediate image is an axial image and wherein for radial image formation at the exit plane the following equation applies:

g, length of the straight part of the flight path adjoining the entrance plane g', length of the straight part of the flight path adjoining the exit plane r radius of the central arcuate path of the ions in the deflection field h Deflection field inhomogeneity parameter I Angle of deflection in deflection field 3. Time-of-flight mass spectrometer according to claim 2, wherein the source supplies ions of substantially equal energies, and wherein the deflection field generating means are electrostatic means generating an electric toroid field having an inhomogeneity parameter h, the total length g of the straight parts of the ion path being given by:

r radius of the central arcuate path of the ions in the deflection field I angle of deflection in deflection field 4. Time-of-flight mass spectrometer according to claim 3 wherein the deflection field generating means comprise a toroid condenser having an aspect ratio of unity (that is, h l) and wherein the flight path has two equal straight parts each equal to 5.9r where D l99.2

5. Time-of-flight mass spectrometer according to claim 3, wherein the flight path has two equal straight parts each equal to 2.35r,,, the deflection field generating means comprising a toroid condenser having an aspect ratio of 0.234 and defining a deflection field with a total deflection angle 1 of l63.2

6. Time-of-flight mass spectrometer according to claim 2 wherein the ion source supplies ions with a substantially equal momentum, and wherein the deflection field generating means are magnetic and define a magnetic sector field with a central radius r and a sector angle I satisfying the equation:

23, 2g, r /h D( l/h l) sin (lz)/lzl the inhomogeneity parameter I! being equal to l-n where n is the inhomogeneity factor of the magnetic field.

7. Time-of-flight mass spectrometer according to claim 6, wherein the inhomogeneity factor n of the magnetic deflection field is equal to 0.5 and wherein the sector angle 1 3l4 and g, g, 3.7r,,

8. Time-of-flight mass spectrometer according to claim 6, wherein inhomogeneity factor n of the magnetic field is equal to 0.5, the sector angle 4 290 and g g, l.23r

9. Time-of-flight mass spectrometer according to claim 6, wherein the magnetic field generating means define two magnetic sectors of equal angle 1? separated by a straight path part and arranged to deflect the beam successively in opposite directions.

10. Time-of-flight mass spectrometer according to claim 1, wherein the following equation applies for axial image formation at the exit plane:

g g',, lengths of the respective straight parts of the flight path adjoining the entrance and exit planes respectively r radius of the central arcuate path of the ions lll the deflection field 1 angle of deflection in deflection field k inhomogeneity parameter of deflection field.

11. Time-of-flight mass spectrometer according to claim 1, wherein the ion flight path includes in succession: a first straight flight part of a length g, r,,/h ctn (11); a first portion of the deflection side producing a deflection angle I the apertured stop being situated symmetrically in said first deflection field portion; a straight-line flight path with the length g (2r cos (h l l/h sin (11 1 a second portion of the deflection field producing the same deflection angle I and a third straight-line flight path of length g whereby the following equations are satisfied, in case of the acceleration of ions to equal momentum:

ll an in the case of the acceleration of ions to equal energies:

where r radius of the central arcuate path of the ions in the deflection field h deflection field inhomogeneity parameter.

l2. Time-of-flight mass spectrometer comprising means defining an ion flight path, having an entrance plane and an exit plane, a pulsed ion source beyond the entrance plane, which source emits a parcel of accelerated ions into said flight path, in which the ion parcel divides into partial parcels of ions of equal effective mass, with the ions of different partial parcels having differing effective mass, said flight path including a straight part, field generating means defining a deflection field which is traversed by the ions in substantially arcuate paths and which is dimensioned with regard to the straight part in such a way that the dispersion in the time of flight of the ions in the deflection field, causd by different initial velocities of ions of equal effective mass, is equal but reverse in sign to that in the straight path, and ion detection means located beyond the exit plane of the flight path, wherein the improvements consists in the following in combination:

7 a. an apertured stop is located at an intermediate image plane in the flight path; b. the flight path includes a straight part and c. the field generating means are electrostatic and define an electrostatic deflection field which is traversed by the ions in substantially arcuate paths and in which the travelling time dispersion of the ions, caused by different initial velocities of equal effective mass, is equal and opposite to that in the straight path, and d. the following equations are satisfied: 3 4r [I (1/h A) sin (h m/I1 r /h tg (180 h I /2) where g total length of the straight part;

h deflection field inhomogeneity parameter r,, radius of the central arcuate path of the ions in the deflection field D angle of deflection in deflection field l3. Time-of-flight mass spectrometer according to claim 12 wherein the electrostatic field generating means comprise a toroid condenser with an aspect ratio of 0.4 producing a deflection angle (1 of 284.5, the length g of the straight part of the flight path is 1.5 r,,.

14. Time-of-flight mass spectrometer according to claim 12, including an ion optical lens located in the straight part of the flight path and producing a stigmatic image at the entrance to the deflection field. 

1. Time-of-flight mass spectrometer comprising means defining an ion flight path having an entrance plane and an exit plane, a pulsed ion source beyond the entrance plane, which source emits a parcel of accelerated ions into said flight path, in which the ion parcel divides into partial parcels of ions of equal effective mass, with the ions of different partial parcels having differing effective mass, said flight path including a straight part, field generating means defining a deflection field which is traversed by the ions in substantially arcuate paths and which is dimensioned with regard to the straight part in such a way that the dispersion in the time of flight of the ions in the deflection field, caused by different initial velocities of ions of equal effective mass, is equal but reverse in sign to that in the straight path, and ion detection means located beyond the exit plane of the flight path, wherein the improvement consists in that the flight path is symmetrical with respect to a plane of symmetry mid-way between the entrance plane and the exit plane, said path including at least two said straight parts adjoining opposite boundaries of the deflection field, and an apertured stop located at an intermediate image plane in said plane of symmetry in the deflection field, said field generating means defining two deflection field areas symmetrically on opposite sides of said plane of symmetry, which areas are traversed by ions on opposite respective sides of a central arcuate path passing through said stop aperture, and means converging the ions to an image in the exit plane under conditions of both velocity and direction focusing.
 2. Time-of-flight mass spectrometer according to claim 1 wherein the intermediate image is an axial image and wherein for radial image formation at the exit plane the following equation applies: gr g''r (ro/h) tg (180* - (h/2) Phi ) where gr length of the straight part of the flight path adjoining the entrance plane g''r length of the straight part of the flight path adjoining the exit plane ro radius of the central arcuate path of the ions in the deflection field h DEflection field inhomogeneity parameter Phi Angle of deflection in deflection field
 3. Time-of-flight mass spectrometer according to claim 2, wherein the source supplies ions of substantially equal energies, and wherein the deflection field generating means are electrostatic means generating an electric toroid field having an inhomogeneity parameter h, the total length g of the straight parts of the ion path being given by: g/4 ro ((1/h2 - 1/4 ) Phi - sin (h Phi )/h3) where ro radius of the central arcuate path of the ions in the deflection field Phi angle of deflection in deflection field
 4. Time-of-flight mass spectrometer according to claim 3 wherein the deflection field generating means comprise a toroid condenser having an aspect ratio of unity (that is, h 1) and wherein the flight path has two equal straight parts each equal to 5.9ro, where Phi 199.2*
 5. Time-of-flight mass spectrometer according to claim 3, wherein the flight path has two equal straight parts each equal to 2.35ro, the deflection field generating means comprising a toroid condenser having an aspect ratio of 0.234 and defining a deflection field with a total deflection angle Phi of 163.2*
 6. Time-of-flight mass spectrometer according to claim 2 wherein the ion source supplies ions with a substantially equal momentum, and wherein the deflection field generating means are magnetic and define a magnetic sector field with a central radius ro and a sector angle Phi , satisfying the equation: 2gr 2g''r ro/h ( Phi (1/h2 - 1) - sin (h Phi )/h3) the inhomogeneity parameter h being equal to 1-n where n is the inhomogeneity factor of the magnetic field.
 7. Time-of-flight mass spectrometer according to claim 6, wherein the inhomogeneity factor n of the magnetic deflection field is equal to 0.5 and wherein the sector angle Phi 314* and gr g''r 3.7ro
 8. Time-of-flight mass spectrometer according to claim 6, wherein inhomogeneity factor n of the magnetic field is equal to 0.5, the sector angle Phi 290* and gr g''r 1.23ro.
 9. Time-of-flight mass spectrometer according to claim 6, wherein the magnetic field generating means define two magnetic sectors of equal angle Phi separated by a straight path part and arranged to deflect the beam successively in opposite directions.
 10. Time-of-flight mass spectrometer according to claim 1, wherein the following equation applies for axial image formation at the exit plane: ga g''a (ro/k) tg (180* - (k/2) Phi ), ga, g''a lengths of the respective straight parts of the flight path adjoining the entrance and exit planes respectively ro radius of the central arcuate path of the ions in the deflection field Phi angle of deflection in deflection field k inhomogeneity parameter of deflection field.
 11. Time-of-flight mass spectrometer according to claim 1, wherein the ion flight path includes in succession: a first straight flight part of a length gr ro/h ctn (h Phi ); a first portion of the deflection side producing a deflection angle Phi , the apertured stop being situated symmetrically in said first deflection field portion; a straight-line flight path with the length g (2r cos (h Phi ) - 1/h sin (h Phi )); a second portion of the deflection field producing the same deflection angle Phi and a third straight-line flight path of length gr, whereby the following equations are satisfied, in case of the acceleration of ions to equal momentum: 1/h ctn (h Phi ) 1/2 ( Phi (1/h2 - 1) - sin (h Phi )/h3 + 1/h sin (h Phi )) an in the case of the acceleration of ions to equal energies: 1/h ctg (h Phi ) 2 ( Phi (1/h2 - 1/4 ) - sin (h Phi )/h3 + 1/4h.sin(h Phi )) where ro radius of the central arcuate path of the ions in the deflection field h deflection field inhomogeneity parameter.
 12. Time-of-flight mass spectrometer comprising means defining an ion flight path, having an entrance plane and an exit plane, a pulsed ion source beyond the entrance plane, which source emits a parcel of accelerated ions into said flight path, in which the ion parcel divides into partial parcels of ions of equal effective mass, with the ions of different partial parcels having differing effective mass, said flight path including a straight part, field generating means defining a deflection field which is traversed by the ions in substantially arcuate paths and which is dimensioned with regard to the straight part in such a way that the dispersion in the time of flight of the ions in the deflection field, causd by different initial velocities of ions of equal effective mass, is equal but reverse in sign to that in the straight path, and ion detection means located beyond the exit plane of the flight path, wherein the improvements consists in the following in combination: a. an apertured stop is located at an intermediate image plane in the flight path; b. the flight path includes a straight part and c. the field generating means are electrostatic and define an electrostatic deflection field which is traversed by the ions in substantially arcuate paths and in which the travelling time dispersion of the ions, caused by different initial velocities of equal effective mass, is equal and opposite to that in the straight path, and d. the following equations are satisfied: g 4ro ( Phi (1/h2 - 1/4 ) - sin (h Phi )/h3) ro/h tg (180* - h Phi /2) 0 where g total length of the straight part; h deflection field inhomogeneity parameter ro radius of the central arcuate path of the ions in the deflection field Phi angle of deflection in deflection field
 13. Time-of-flight mass spectrometer according to claim 12, wherein the electrostatic field generating means comprise a toroid condenser with an aspect ratio of 0.4 producing a deflection angle Phi of 284.5*, the length g of the straight part of the flight path is 1.5 ro.
 14. Time-of-flight mass spectrometer according to claim 12, including an ion optical lens located in the straight part of the flight path and producing a stigmatic image at the entrance to the deflection field. 